1. Field of the Invention
The present invention relates to a plate-like member conveying apparatus.
2. Description of the Related Art
In semiconductor manufacturing steps and the like, various types of plate-like member conveying apparatuses have been used as an apparatus for conveying plate-like members such as semiconductor wafers.
For example, Published Unexamined Japanese Patent Application No. 64-35746 discloses a plate-like member conveying apparatus in which five arms for supporting semiconductor wafers are arranged such that semiconductor wafers are vertically arranged at equal intervals to be substantially parallel to each other. In this apparatus, a plurality of ball screws having different leads and directions are formed on a rotatable shaft, and nuts are arranged on the proximal end portions of the arms to oppose these ball screws. The proximal end portions of the arms are respectively attached to the ball screws. When the shaft is rotated, the arms are translated to change the pitch of the semiconductor wafers.
In such a plate-like member conveying apparatus, a transfer operation can be performed while the pitch of a plurality of semiconductor wafers is changed. For example, the apparatus is used to transfer semiconductor wafers between a wafer cassette and a wafer boat for a heat-treatment apparatus. The transfer operation is automatically performed in accordance with the control of the program previously set in the computer.
In the above-described conventional plate-like conveying apparatus, however, since metal ball screws and metal nuts which are engaged with each other are used, if the shaft is rotated a large number of times to change the interval between plate-like members, the ball screws and the nuts are brought into slidable contact with each other to generate a large amount of dust, e.g., metal dust and the mist of a lubricating oil. Such dust is scattered, and some of the scattered dust adheres to the surfaces of semiconductor wafers, resulting in an increase in rejection rate of semiconductor devices. Especially in recent years, with an increase in integration density of a semiconductor device, there has been a great need to suppress the generation of even only a little dust. Therefore, the generation of dust poses a serious problem.